Developers of information storage devices continue to seek increased storage capacity. As part of this development, holographic memory systems have been suggested as alternatives to conventional memory devices. Holographic memory systems may be designed to record data one bit of information (i.e., bit-wise data storage). See McLeod et al. “Micro-Holographic Multi-Layer Optical Disk Data Storage,” International Symposium on Optical Memory and Optical Data Storage (July 2005). Holographic memory systems may also be designed to record an array of data that may be a 1-dimensional linear array (i.e., a 1×N array, where N is the number linear data bits), or a 2-dimension array commonly referred to as a “page-wise” memory systems. Page-wise memory systems may involve the storage and readout of an entire two-dimensional representation, e.g., a page of data. Typically, recording light passes through a two-dimensional array of dark and transparent areas representing data, and the system stores, in three dimensions, the pages of data holographically as patterns of varying refractive index imprinted into a storage medium. See Psaltis et al., “Holographic Memories,” Scientific American, November 1995, where holographic systems are discussed generally, including page-wise memory systems.
In a holographic data storage system, information is recorded by making changes to the physical (e.g., optical) and chemical characteristics of the holographic storage medium. These changes in the holographic storage medium take place in response to the local intensity of the recording light. That intensity is modulated by the interference between a data-bearing beam (the data beam) and a non-data-bearing beam (the reference beam). The pattern created by the interference of the data beam and the reference beam forms a hologram which may then be recorded or written in the holographic storage medium. If the data-bearing beam is encoded by passing the data beam through, for example, a spatial light modulator (SLM), the hologram(s) may be recorded or written in the holographic storage medium as holographic data.
External cavity laser diodes (ECLDs) are useful light sources for applications in spectroscopy, telecommunications and holography. Holographic data storage also illustrates an application with three requirements that an ECLD meets: wide wavelength tuning range, operation in a single-longitudinal mode, and output powers in the tens of milliwatts. In some holographic data storage approaches, the operating wavelength range may be in the range of from about 402 to about 408 nm. Because holograms are created by interference, single-longitudinal mode operation may be necessary to form holograms having a high signal-to-noise ratio. Finally, the strength of the created holograms, which are stored in a holographic storage medium, depends upon the number of photons delivered to the storage medium. (The number of photons absorbed by the holographic storage medium determines how much of a photochemical change occurs which creates the modulations in the index of refraction of the medium, which is how the holograms are stored.)